1. Field of the Invention
The present invention relates to a light emitting panel in which a light emitting element formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to a light emitting module in which an IC or the like is mounted on the light emitting panel. Note that, in this specification, the light emitting panel and the light emitting module are generically called light emitting devices. The present invention further relates to electronic apparatuses utilizing the light emitting devices.
2. Description of the Related Art
A light emitting element emits light by itself, and thus, has high visibility. The light emitting element does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the light emitting element has no limitation on a viewing angle. Therefore, the light emitting device using the light emitting element has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
Incidentally, the light emitting element means an element of which a luminance is controlled by electric current or voltage in this specification. The light emitting element includes an OLED (organic light emitting diode), an MIM type electron source element (electron emitting elements) used to a FED (field emission display) and the like.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. These layers may have an inorganic compound therein. The OLED basically has a structure in which an anode, a light emitting layer, a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode, a hole injecting layer, a light emitting layer, a cathode are laminated in order or a structure in which an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, a cathode are laminated in order.
On the other hand, the decreased luminance of OLED resulting from the deterioration of the organic light emitting material poses a serious problem on the practical use of the light emitting devices.
FIG. 21A graphically illustrates a time-varying luminance of the light emitting element when a constant current is applied between the two electrodes thereof. As shown in FIG. 21A, the luminance of the light emitting element decreases despite the application of the constant current because the organic light emitting material is deteriorated with time.
FIG. 21B graphically illustrates a time-varying luminance of the light emitting element when a constant voltage is applied between the two electrodes thereof. As shown in FIG. 21B, the luminance of the light emitting element decreases with time despite the application of the constant voltage. This is partly because, as shown in FIG. 21A, the deterioration of the organic light emitting material entails the decrease of the luminance at the constant current and partly because the current flow through the light emitting element caused by the constant voltage is decreased with time, as shown in FIG. 21C.
The decreased luminance of the light emitting element with time can be compensated by increasing the current supply to the light emitting element or increasing the voltage applied thereto. In most cases, however, an image to be displayed includes gradation levels varying from pixel to pixel so that the individual light emitting elements of the pixels are deteriorated differently, resulting in the variations of luminance. Since it is impracticable to provide each of the pixels with a power source for supplying voltage or current thereto, a common power source for supplying the voltage or current to all the pixels or a group of some pixels. Therefore, if the voltage or current supply from the common power source is simply increased to compensate for the decrease in the luminance of some light emitting elements due to deterioration, all the pixels supplied with the increased voltage or current are uniformly increased in luminance. Hence, the luminance variations among the individual light emitting elements of the pixels are not eliminated.